Contention-based multiple access protocols use carrier sensing and back off mechanisms to reduce the probability of collision. For example, carrier sense multiple access (CSMA) is a probabilistic media access control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum. Carrier sense generally means that a transmitter uses feedback from a receiver to determine whether another transmission is in progress before initiating a transmission. That is, carrier sense tries to detect the presence of a carrier wave from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission. In other words, CSMA is based on the principle “sense before transmit” or “listen before talk”. Multiple access generally means that multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations connected to the medium.
In consequence, a transmitting device normally listens to the channel for a minimum amount of time. If the channel is sensed to be occupied by another transmission, the transmitting device defers its transmission to a future time.
During this time period from back off until the future time is reached, the device typically continues monitoring the status of the channel, thus consuming energy.